Interesting discussion.
I looked into this a bit for my book _Soldiers' Lives through History: The Middle Ages_, and here's what I came up with:
When French or English kings called up the arrière-ban—the full mobilization of military manpower for national defense, based on sovereign rather than feudal or contractual authority—the summons was generally directed at all those 16-60, but there are many examples of younger boys or older greybeards campaigning. [83] In the modern West, and especially in America, we have become accustomed to the idea of soldiering as mainly the occupation of very young men (and now women), mainly in the 18-22 range. Across the Middle Ages, the attitude towards “military age” seems to have been quite different. Although it was common to take up arms as early as 15 (and sometimes younger, especially among the higher elite), it was more typically only around the age of 20 or 21 that one was considered fully of mature fighting age, and, as among the ancient Greeks, a warrior’s prime was thought to come much later, somewhere around 35. [84] In the fifteenth-century Petit Jean de Saintré, for example, when the protagonist undertakes his first “deed of arms” at the age of twenty or twenty-one, it is repeatedly and heavily stressed that he is “yet but a child in years,” and his opponent, old enough to be his father, is dismayed by the mismatch, not because he thinks Petit Jean’s youthful vigor will be an unfair advantage, but because he thinks he cannot gain much credit for defeating someone of so little experience, only “an apprentice to arms.”[85] And Díez de Games notes with approval the advice supposedly given by Aristotle to Alexander: “when you are ready to raise your army, you should not leave behind the old in order to take along the young: the old give sound counsel, which is valuable in a fight, and once they have taken their position on the battlefield, they will not allow themselves to be forced from it.” [86].
Notes:
[83]: 16-60: E.g. Wars of Edward III, 118; PRO C47/2/60 mm. 34-41 (England); O’Cléirigh, 186 (Anglo-Irish); similarly Winkler, Swiss and War 105 (Switzerland); 16-65 on Malta (Theresa M. Vann, “The Militia of Malta,” JMMH 2 [2004], 144); 15 (or 16)-59 on Sicily (Muntaner, Chronicle [In Par. ed.], 117); 15-60 in Reims’ watch (Michael [C.] Jones, "War and Fourteenth-Century France," in Anne Curry and Michael Hughes, eds. Arms, Armies and Fortifications in the Hundred Years War [Woodbridge: Bodyell Press, 1994], 117); 18-60 at Gisors and in Midi urban militias generally (Delachenal, Hist. Charles V, 185 n. 1; Noël, Town Defence, 102), etc. In the Liberties conceded to Barcelona in 1283, service was required up to age 70. (Text cited in Du Cange, s.v. hostis, p. 246, col. 2.) The “hoar-headed” leader in battle is a staple of medieval literature (e.g. in The Battle of Maldon; note also what appear to be several graybeards among the Anglo-Saxons depicted on the Bayeux Tapestry) and supported by many documented instances of men campaigning into their sixties and later (e.g. William the Marshal at Lincoln in 1217, and Talbot at Castillon in 1453).
[84]: In the Early Medieval period, “it seems that the age at which a boy was expected to begin to perform military service was about fifteen. Yet, in the furnished cemeteries of the period, the age around twenty was symbolised by the acquisition either of new and more numerous types of weaponry or, in some areas, of weaponry of any kind.” Halsall, Warfare, 35 (and more generally 33-36). In the late 14th century, similarly, “fourteen or fifteen was a very common age for first arming, and it was not exceptional to be armed even younger: but…there were a good many who did not see service until their twenties." Maurice H. Keen, "English military experience and the court of chivalry: the case of Grey v. Hastings." in Philippe Contamine, et al. (eds.), Guerre et société en France, en Angleterre et en Bourgogne. XIVe-XVe Siècle. (Lille: Centre d'histoire de la region du nord et de l'Europe du nord-ouest, 1991), 131; also Prestwich, Armies, 54-5. The Black Prince fought at Crécy at 16, but at the same age Raymond of Toulouse (a hundred and forty-three years earlier) was considered “unfitted for fighting because of his age.” Peter of Vaux-de-Cernay, History of the Albigensian Crusade, tr. W. A. Sibly and M. D. Sibly (Woodbridge: Boydell, 1998), 332. In Spanish urban charters, the minimum age for mandatory military service ranged from 13 to 20. Powers, Society Organized for War, 122-3. Anna Comnena (Alexiad, 326) describes elite young soldiers “full [of] physical vigour, with beards scarcely grown”—but then emphasizes their skill with bow and thrown spear, rather than in the line of battle. Diez de Gamez considered that Pero Niño, who first received arms and engaged in combat at around 15 (which was early, due to circumstances) was a “youth” until 25, “the age of manhood.” Unconquered Knight (In Par. ed.), 13, 21. Wace describes Emperor Lucius as in the flower of his age at “more than thirty, but less than forty” which matches Froissart’s opinion that Edward III was “in the flower of his youth” in 1346, at 33, and likewise the Black Prince was “in the prime [droite] flower of his youth” when he invaded Castile, at the age of 36. Wace, The ‘Arthurian’ Portion of the Roman de Brut, tr. Eugene Mason (Cambridge, Ontario: In Parentheses Publications, 1999), online at
http://www.yorku.ca/inpar/Arthurian.html, p. 105; Froissart, Oeuvres, 4:381; Froissart, Chronique (ed. Sanderson), 324.
[85]: de la Sale, Little John of Saintré, 107, 109, 116, 123, 124, 136-7; note also James, Book of Deeds, 101; Diaz de Gamez, Unconquered Knight (In Par. ed.), 14-15 (when Pero Niño was about 18, and had a household and men under him, but gave such blows that those who fought him “thought they had to do not with a youth, but with a man robust and grown.”)
[86]:Gutierre Diaz de Gamez, El Victorial; crónica de don Pero Niño, conde de Buelna, ed. Juan de Mata Carriazo (Madrid: Espasa-Calpe, 1940), 14 (omitted from the English translation). Note also Las Siete Partidas, tr. Samuel Parsons Scott, ed. Robert I. Burns (Philadelphia: Philadelphia University Press, 2001), II.19.3 (wise old men are “most necessary” in an army), and Maurice, Strategikon, 29 (squads should mix strong young men with steady old ones.)